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Draw and Identify Right, Acute, and Obtuse Angles

Students classify angles as right (exactly 90°), acute (less than 90°), or obtuse (greater than 90° but less than 180°), draw examples of each type, and identify these angle types in two-dimensional figures and real-world objects.

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Draw and Identify Right, Acute, and Obtuse Angles

Grade 4 · Lines and Angles in Two-Dimensional Figures · question 1 of 3

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20 rungs. A star at the top.

ABC

At A in triangle ABC, which angle type do you see?

Look at the marked corner.

How it is taught

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A right angle matches a square corner.

A narrower opening is less than 90.

LMN

Read it: Hold the angle against a square corner

Angle PQR opens wider than a square corner, but it is not a straight line, so it is obtuse.

  1. Put a square corner on the vertex, along one ray.
  2. See where the other ray falls: inside the corner, on its edge, or outside it.
  3. Name the angle: inside is acute, on the edge is right, outside is obtuse.

A square corner is the right angle every other angle is held against.

An acute angle opens less than a square corner, and an obtuse one opens more.

The kind depends on the opening between the rays, not on how long the rays are drawn.

Narrower than a square corner is acute, a square corner is right, and wider is obtuse.

Calling angle PQR acute because its rays are drawn short.

Short rays do not make a small angle. Angle PQR opens wider than a square corner, so it is obtuse.

An angle measures 150°. What kind of angle is it? — an obtuse angle. 150° is more than 90° and less than 180°, so it is obtuse.

Classify three angles by comparing to 90°

Today we'll learn three types of angles. You'll spot them in shapes everywhere!

Here's a right angle. It looks like a perfect corner — like the corner of a book.

Grade 4 · Three kinds of angles

We mark right angles with a tiny square in the corner, so everyone knows.

A right angle measures exactly ninety degrees.

Here's an acute angle. It opens up smaller — sharper, like a slice of pizza.

An acute angle is less than ninety degrees.

Here's an obtuse angle. It opens up wider, past the square corner.

An obtuse angle is greater than ninety degrees, but less than one hundred eighty.

Look at all three together! Acute is sharp, right is square, obtuse is wide.

So remember: compare to ninety degrees. Smaller is acute, equal is right, bigger is obtuse!

Now you try. Read this protractor. What is the measure?

What you see

While they climb, your page writes one true line to ask about tonight, and a short note each Sunday.

MayaGrade 3 · Mathematics
Did you know?

A chessboard has 64 squares — share them out between the 8 rows and each row holds exactly 8.

Today’s goal
1 star — one skill mastered, every rung of its ladder climbed
Climbing now
Divide within 100

An example student.

MayaGrade 3 · Mathematics

Maya earned 2 stars this week — Interpret multiplication as equal groups, which took 6 answers over 3 days before its star landed 3 days ago and Multiply by 6, 7, 8 and 9. Next up: Divide within 100.

An example week.

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